Ethanol, the main ingredient in alcoholic beverages, has been consumed since the development of alcoholic beverages. However, in modern times, ethanol is attracting considerable attention as a clean energy alternative to petroleum. In Brazil, ethanol has been commercialized as a fuel for transportation for a long time. Ethanol fuels, consisting of 10% ethanol and 90% gasoline, are commercially available in the U.S.A.
Meanwhile, at the conventional low-cost oil age involving cheap petroleum, the cost of manufacturing ethanol is higher than the price of oil, and thus ethanol is not cost-competitive. However, taking into consideration gradual exhaustion of oil deposits and increased oil prices, ethanol is becoming increasingly cost-competitive and will eventually overtake the price competitiveness of petroleum.
Ethanol used as a transportation fuel is prepared from sugarcane, corn, etc. Sugarcane is a raw material of sugar and corn is a raw material of foods. For this reason, the use of these sources for the preparation of ethanol causes, as a side-effect, an increase in sugar or corn costs and, as an ethical problem, use of the grain for fuel materials, rather than food materials.
Accordingly, a great deal of research is widely conducted on the development of alternatives to sugarcane and corn. Xylose, found at high concentrations in waste wood materials or forestry by-products, is one potential candidate.
Xylose, a form of biomass, is found at high concentrations in wood waste in the form of polymeric xylene. Xylose is separated from xylo-oligosaccharide (XOS) formed through hydrolysis of xylan, which is used for the preparation of xylitol capable of preventing dental caries.
Xylose is a renewable resource, which is recovered from wood byproducts generated in the process of producing pulp, etc. Use of xylose does not entail an increase in alternative material prices and is free from ethical problems and a great deal of research associated therewith is thus conducted at present.
Meanwhile, Saccharomyces cerevisiae is well-known as a strain for producing ethanol in the preparation of fermented liquor such as coarse liquor. Saccharomyces cerevisiae is actively utilized as a host for the preparation of useful medicines and its use as a host for preparing ethanol is actively researched.
Saccharomyces cerevisiae has neither xylose reductase (XR) nor xylitol dehydrogenase (XDH) and thus, disadvantageously, cannot metabolize xylose.
Accordingly, in order to allow Saccharomyces cerevisiae to metabolize xylose, a great deal of research is being conducted into incorporation of these enzymes into Saccharomyces cerevisiae. From research results, it can be confirmed that xylose is substantially metabolized.
In addition, when Saccharomyces cerevisiae metabolizes xylose, ethanol is produced as a byproduct. Accordingly, some researchers have attempted to produce ethanol coming into the spotlight as an alternative energy using Saccharomyces cerevisiae. 
In this regard, the preparation of ethanol from xylose causes problems of low preparation yield and low economic efficiency. These problems are also caused by limited supply of NAD(P)H and NAD (P)+, as cofactors, and acetic acid obtained as a by-product.